There are a lot of tools for different kinds of filtration jobs:
* Filter Socks/Sponges - mechanical filtration, must be changed regularly
* Live Rock/Sand - provides growth media for nitrifying bacteria (exports/processes ammonia through nitrogen cycle)
* Protein Skimming - nutrient export via foam fractionation (exports nitrates and other nutrients)
* Activated Carbon - binds dissolved organic compounds and polishes the water (best in a media reactor)
* Granular Ferric Oxide (GFO) - binds phosphate (best in a media reactor)
I run all of these in our sumps, but you should consider what the problem is you're trying to solve and find a solution to match it.